Multimedia projection systems have become popular for purposes such as conducting sales demonstrations, business meetings, and classroom training. In typical operation, multimedia projection systems receive analog video signals from a personal computer and convert the video signals to digital information to control one or more digitally driven light valves. Depending on the cost, brightness, and image quality goals of the particular projector, the light valves may be of various sizes and resolutions, be transmissive or reflective, and be employed in single or multiple light path configurations.
Recently, more optimal sets of multimedia projector characteristics have been achieved by employing reflective LCOS light valves. There are five general architectures for employing reflective LCOS light valves. The first employs a polarization beam splitter (“PBS”) cube prism and a so-called Philips prism; the second employs a PBS cube prism, a dichroic prism, and spectrally selective wave plates; the third employs multiple PBS cube prisms; the fourth employs a PBS cube prism and tilted plates; and the fifth employs an off-axis design implemented with linear polarizers, as opposed to PBS cube prisms. For each architecture, a number of variations exist, such as using crossed plates for color separation versus a solid “X cube” prism color separator, using liquid filled PBS cubes instead of glass PBS cubes, and using additional polarizers or wave plates. However, each of the five architectures is generally distinct from the others and from the invention described herein.
All of the above architectures employ linear polarized light-sensitive devices for receiving light from a light source, reflecting the light off the LCOS light valves, and redirecting the reflected light, depending on its polarization direction, either out through a projection lens or back toward the light source. The polarization direction of the light is determined by an electronic image pattern applied to the LCOS light valve. To achieve a dark polarization direction, selected LCOS light valve pixels do not change the reflected light polarization direction, so the light returns to the light source and does not project toward the screen. To achieve an illuminated polarization direction, selected LCOS light valve pixels rotate the polarization direction by 90°, so the light is directed through the projection lens toward the screen. Projected image quality largely depends on how well the various optical path components establish, maintain, and analyze the light polarization directions. Image brightness largely depends on minimizing light loss through the various optical path components and polarizers.
In particular, the architecture employing a PBS cube prism and a Philips prism is described in U.S. Pat. No. 5,777,789 for EFFICIENT OPTICAL SYSTEM FOR A HIGH RESOLUTION PROJECTION DISPLAY EMPLOYING REFLECTION LIGHT VALVES, in which a cube PBS allows only linearly polarized light to propagate to a color splitting/combining prism. After reflecting from the light valves, the light is “analyzed” by the PBS cube and redirected according to the polarization direction of the analyzed light. This architecture is disadvantageous because it requires sophisticated optical coatings and non-standard prism angles and has skew ray depolarization caused by the PBS cube prism, stress birefringence caused by long path lengths in glass elements, and considerable weight due to the bulky prisms.
In the architecture employing a PBS cube prism, a dichroic prism, and spectrally selective wave plates, linearly polarized light is first incident on a spectrally selective half-wave plate that changes the polarization direction by 90° for one color band only. A PBS cube separates the rotated color band from the unrotated color bands based on their orthogonal polarization directions. Typically the green band is selected as the rotated color band because a dichroic cube splitter relatively easily separates the widely spaced wavelengths of the blue and red bands. After reflection from the light valves, the PBS cube analyzes the light, directs it according to its polarization direction, and recombines the color bands. Because the PBS cube has a non-ideal spectral response, a spectrally selective half-wave plate is required at the output face of the PBS cube so that all three color bands have the same polarization direction after passing through the wave plate and can, therefore, all pass through a “clean-up” polarizer. This architecture is disadvantageous because of stress birefringence caused by the large path lengths in glass, skew ray depolarization caused by the PBS cube prism, and considerable weight due to the bulky prisms.
In the architecture employing multiple PBS cube prisms, light is separated into red, green, and blue light paths using dichroic filter plates. Each of the three color paths contains a PBS cube, and each PBS cube allows only linearly polarized light to pass through to an associated light valve. Light reflected from the light valves is “analyzed” by the respective PBS cube and redirected according to the polarization direction of the analyzed light. For each color path, light propagating toward the projection lens is recombined with light from the other color paths via an X-cube prism. This architecture is disadvantageous because of considerable aggregate weight of the three PBS cube prisms and the X-cube prism, high component cost and complexity, stress birefringence, skew ray depolarization in the PBS cube prisms, and a large footprint created by the separated color paths.
In the architecture employing a PBS cube prism and tilted plates, the PBS cube prism allows only linearly polarized light to propagate toward a set of tilted dichroic filter plates. The first plate reflects one color band and passes the remaining light to the second dichroic filter plate, where it is further split into two more color bands. After reflection from the light valves, the color bands of light retrace their paths and recombine via the color splitting plates. The light is subsequently “analyzed” by the PBS cube, and redirected according to the polarization direction of the analyzed light. This architecture is disadvantageous because the PBS cube prism is bulky, heavy, has stress birefringence, and skew ray depolarization, and the projection lens requires a long back working distance.
The architecture employing an off-axis design and linear polarizers is described in “projection Displays V,” SPIE Proceedings, January 1999, Vol. 3634, pp. 80-86. This architecture employs a two-level arrangement in which the incoming light propagates upwardly at an angle and through crossed dichroic color splitting plates. A sheet type linear polarizer positioned in each color path polarizes the light. The polarized light continues to propagate upwardly and reflects off the light valves. The polarization direction of the light is analyzed by another sheet type linear polarizer in each color path Light reflected by dark polarization direction pixels undergoes absorption in the polarizer, and light reflected by illuminated polarization direction pixels propagates through the polarizer to an X-cube prism color combiner. This architecture is disadvantageous because it has an unduly high-profile, two-level form factor and requires an proprietary, asymmetrical, off-axis projection lens.
What is needed, therefore, is a compact, low-profile multimedia projection system that achieves a bright, high-quality projected image at a relatively low cost.